The present invention is directed to microwave interference, and more particularly to a means of reducing microwave interference among a networked plurality of wireless communication devices.
Devices using the emerging Personal Area Network (PAN) protocols including Bluetooth HomeRF or 802.11 protocols occupy the same radio frequency spectrum as do microwave ovens. Bursts of RF interference from the microwave oven occurs with a repetition rate equal to that of,the alternating current (ac) line frequency supplying the oven. In microwave ovens in particular, a magnetron is typically supplied by the mains directly, via an autotransformer, with no supply filtering. The resulting RF bursts occur only during one half of the ac cycle. Radiation from microwave ovens may be quite strong (in fact equal in strength to that of a Bluetooth device itself), and quite broadband, covering the entire 2.45 GHz ISM band used by Bluetooth devices. This broadband interference covers all possible Bluetooth channels, so that Bluetooth""s principal interference-avoidance strategy (frequency-hopping) is in this case ineffective. Both synchronous voice (SCO) and asynchronous data (ACL) Bluetooth communication links are affected. During half of the ac cycle the SCO voice packets are simply lost, resulting in significantly reduced audio quality, while the ACL links suffer multiple NACKs and reduced data throughput as the links are lost and then reestablished each ac cycle.
A primary market for personal area networks and particularly Bluetooth is at-home use by consumers, as either a cordless telephone system or a home wireless network connecting intelligent appliances. In either of these applications the loss of the personal area network while a microwave oven is on is clearly unacceptable; similarly, operation of the microwave oven cannot be prohibited during use of the personal area network. Thus, a solution is needed to allow microwave ovens and Bluetooth networks to coexist.
A relevant FCC regulation for Bluetooth, Code of Federal Regulations, Title 47, Sec. 15.247, states in paragraph (h): xe2x80x9cThe incorporation of intelligence within a frequency hopping spread spectrum system that permits the system to recognize other users within the spectrum band so that it individually and independently chooses and adapts its hopsets to avoid hopping on occupied channels is permitted.xe2x80x9d
Use of algorithms for detecting periodicity on desired signals such as baud detection algorithms is well known. The present invention utilizes a baud detection algorithm in a novel way to identify interference having a particular periodicity (50 or 60 Hz) indicative of microwave oven interference. Conventional noise blankers, which typically blank receiver audio output when a measured noise level exceeds a threshold, are also known in the art, but they differ from the present invention in that they are essentially xe2x80x9creal timexe2x80x9d devices without memory, and do not distinguish between interference occurring at different repetition rates.
U.S. Pat. No. 5,838,741, (entitled xe2x80x9cCommunication Device and method for reducing effects of noise introduction by synchronizing data transfer to a received signal,xe2x80x9d by Callaway, Ansari, Mock, Eaton, and Hayes, issued Nov. 17, 1998) describes an interference avoidance method in which noise-producing activities are timed to occur during periods of the desired signal that are not sensitive to noise (e.g., symbol transitions). The present invention differs from U.S. Pat. No. 5,838,741 in that, in the present invention, the desired system operation is synchronized to existing, externally generated, periodic noise.
U.S. Pat. No. 6,006,071 (entitled xe2x80x9cRF Communication System Operable in the Presence of a Repetitive Interference Source and Related Methodsxe2x80x9d) that not only appears to require forward error correction, but also requires the sending of data packets twice. ""071 describes a one-way system for wireless speakers where no attempt is made to determine which half-cycle is causing the interference. In essence, data packets are sent twice in synchrony with the AC cycle so that one of the data packets will be in the clear half-cycle. ""071 also uses AC sensing external to the oven/appliance, and so it is possible that the sensor and the oven are on different phases of the AC supply (most houses are fed with multi-phase AC power). In the present invention, current would be sensed going to the magnetron by phase detector 14 in FIG. 1, eliminating this phase uncertainty. Additionally, in the present invention, any communication between the microwave oven and other devices in a piconet would be synchronous to power supplied to the magnetron, rather than the AC mains themselves as shown in ""071.
U.S. Pat. No. 5,574,979 requires tracking an alternating current associated with a power main, whereas the present invention just detects and tracks the periodic interference alone, and is thus suitable for portable battery powered devices.